1. Technical Field
The present invention relates to a wiring board with lead pins and a method of producing it, and more particularly to a wiring board with lead pins in which the bonding strength of the lead pins with respect to a wiring board is improved, and a method of producing the wiring board.
2. Related Art
A wiring board with lead pins is formed by bonding the lead pins to connection pads formed on a wiring board by using a bonding conductive material such as a solder. In a wiring board with lead pins which is formed by bonding lead pins to connection pads by using a solder or the like, usually, lead pins in each of which a flat disk-like head portion is formed integrally with one end of a shaft portion into a nail-like shape are used.
FIGS. 7A and 7B show a conventional method of bonding lead pins 20 to a wiring board 10.
FIG. 7A shows a state where the lead pins 20 are set in a pin bonding jig 30. In the jig 30, set holes 30a into which shaft portions 20a of the lead pins 20 are to be passed and set in a planar arrangement that is identical with that of connection pads of the wiring board are formed, and set recesses 30b which communicate with the set holes 30a, respectively, and in which head portions 20b of the lead pins 20 are to be housed, respectively are formed. When the lead pins 20 are to be set to the jig 30, a distributing machine for the lead pins 20 is used, and the lead pins 20 are distributed to all of the set holes 30a of the jig 30.
FIG. 7B shows a state where the jig 30 in which the lead pins 20 are set is positioned to the wiring board 10, and the lead pins 20 are bonded to connection pads 12 of the wiring board 10 by solder reflow. As a conductive material for bonding, for example, a solder 14 is previously supplied to the connection pads 12. The jig 30 is passed through a reflow apparatus in a state where the lead pins 20 are supported by the jig, whereby the lead pins 20 are bonded to the connection pads 12.
In the jig 30, the set holes 30a and the set recesses 30b have a slight clearance with respect to the shaft portions 20a and the head portions 20b in order that the lead pins 20 are set by the distributing operation, and, after the lead pins 20 are bonded to the connection pads 12, the jig 30 is upward pulled out. For the sake of description, the clearances are exaggeratingly shown in FIGS. 7A and 7B.    [Patent Reference 1] JP-A-2001-148441    [Patent Reference 2] JP-A-2000-58736
In the above-described wiring board with lead pins which is formed by bonding the lead pins 20 to the wiring board 10, the bonding strength between the lead pins 20 and the connection pads 12 depends on the bonding force by which the head portions 20b are bonded to the connection pads 12. In each of the lead pins 20, the head portion 20b which is larger in diameter than the shaft portion 20a is formed because the bonding area with respect to the solder 14 (conductive material) is ensured by using the end area of the head portion 20b, thereby enabling a predetermined bonding strength (pull strength) to be obtained.
As a method of enhancing the bonding strength of the lead pin 20, a method in which the shape of the head portion 20b of the lead pin 20 is improved, and that in which a basal portion of the lead pin 20 is held by an adhesive resin material have been proposed. However, sufficient bonding strength is not always obtained. In a wiring board with lead pins, particularly, the diameter of the lead pins 20 is becoming smaller because the number of the pins tends to increase. Therefore, a wiring board with lead pins in which, even in the case where lead pins of a small diameter are used, sufficient bonding strength is obtained is requested.